Conventionally, as a walking crane which is one type of cranes, a walking crane including a crane body and a boom serving as a raising member has been known. The boom is mounted on the front portion of the crane body to be raised or lowered. A cargo is lifted by a wire hung from the distal end of the boom. The long boom warps by some degree by its own weight. When the cargo is lifted with the boom warped, a compression force is applied to the boom along the longitudinal direction of the boom. This results in a large bending moment in the middle portion of the boom, which requires the boom to have high strength.
JP S59-15686 U discloses a crane including a jib serving as a raising member pivotably supported by the distal end of the boom. A guyline for raising and lowering the jib is connected to the distal end of the jib. A mid-support line is provided to connect the middle portion of the guyline and the middle portion of the jib. The mid-support line reinforces the supporting structure of the jib to suppress the warp of the jib.
DE 20215179 U1 discloses a crane including a boom, a mast for raising and lowering the boom, and a rope-shape guyline connecting the distal end of the boom and the distal end of the mast. The guyline extends from the distal end of the mast to the distal end of the boom to run about a sheave provided at a mid-branch-point and then to the middle portion of the boom. The guyline runs about a mid-sheave provided on the middle portion of the boom and back to the sheave at the mid-branch-point and is connected to the distal end of the boom.
In the art disclosed in JP S59-15686 U, each of both ends of the mid-support line is respectively secured to the jib and the guyline. The degree of warp of the jib changes with the raised angle of the jib and the weight of a cargo. Thus, with both the ends of the mid-support line secured, the degree of warp of the jib disadvantageously cannot be suppressed to a minimum for different raised angles of the boom and different weights of the cargo.
In the art disclosed in DE 20215179 U1, the distance between the sheave at the mid-branch-point and the mid-sheave on the boom changes as the tension of the guyline increases or decreases to raise and lower the boom. For this reason, the warp of the boom cannot be corrected by adjusting the distance between the sheaves while the raised angle of the boom is kept constant. Disadvantageously, the degree of warp of the boom cannot be suppressed to a minimum for different raised angles of the boom and different weights of the cargo.